fix(textarea): fix data corruption after multi-line input

Previously, when inserting multiple lines of input at once, the underlying
Go data array was incorrectly shared in memory across multiple rows
of the textarea. This was causing input corruption, where a user trying
to modify one line would see their text added on the next line as well.

This patch fixes it.

textarea/textarea.go

@@ -321,7 +321,11 @@ func (m *Model) insertRunesFromUserInput(runes []rune) {
 	lstart := 0
 	for i := 0; i < len(runes); i++ {
 		if runes[i] == '\n' {
-			lines = append(lines, runes[lstart:i])
+			// Queue a line to become a new row in the text area below.
+			// Beware to clamp the max capacity of the slice, to ensure no
+			// data from different rows get overwritten when later edits
+			// will modify this line.
+			lines = append(lines, runes[lstart:i:i])
 			lstart = i + 1
 		}
 	}